This is a very high yield episode to help you review for the anesthesia board exam. I cover the board key word section entitled gas laws which includes a focus on volatile anesthetics, their properties, and how they act on, and in, the human body.
Topics covered will include:
Which inhaled anesthetic gas has the highest vapor pressure?
C: Nitrous oxide
What is VP proportional to?
Vaporizer Types and safety features
If you accidentally filled your sevo vaporizer with iso, what would happen? Why?
Why does desflurane have its own special type of vaporizer?
What happens when you use a variable bypass vaporizer at altitude?
Why would Des be different at altitude?
Uptake and elimination
Most important concept?
What effect does a R→L intracardiac shunt have on the rate of inhaled induction of anesthesia?
Why more effect on poorly soluble agents?
Additional intracardiac shunt effects (19:50)
What is the blood-gas solubility coefficient?
Effects of ventilation and circulation
What effect does increased alveolar ventilation have on inhaled induction?
More affect on soluble or insoluble?
How is solubility of gas affected by endobronchial intubation?
How does cardiac output affect onset of inhaled induction?
Why inhaled induction so fast in infants?
Second gas effectWhat is it? (28:49)
Nitrous Oxide in closed spacesCommon examples (29:35)
Leave questions or comments here or email email@example.com
Corresponding ppt slides: Gas_Laws
Supplemental explanation of some of these topics by Andrew Hershey: Partial pressure analogy
21 thoughts on “Episode 1: Properties and laws of volatile anesthetics”
This is a great podcast that helps solidify my learning and generate critical thinking. I’m a SRNA and often find myself referring my classmates to this podcast. I just wish there were more! Thank you!
Just stumbled upon your podcast on my commute and wanted to say thank you. It’s great to listen to the discussions while on the road. I just wanted to suggest a correction. My understanding, and I referenced Miller’s Anesthesia to be sure, but Desflurane uses a Tec 6 vaporizer and not a Tec 9 as mentioned in your podcast. My class actually came up with an analogy to help remember this little fact. We call Desflurane the Duke Blue Devil. Blue because of the color coding for Des. Devil because everything involving him surrounds the satanic number 6. For example, MAC = 6%. VP = 666 mmHg, B:G ratio = 0.42 (4+2 = 6), O:G ratio = 18 (3 x 6) and it’s enclosed in a Tec 6 vaporizer.
Hi Patrick, you’re absolutely right. I don’t know where I got tech 9 from but thanks for pointing out the correction!
I wanted to take a moment to sincerely thank you for all the effort you put into ACCRAC. You are a wonderful teacher & I’ve found ACCRAC to be invaluable. Thank you.
Thanks so much, that’s great to hear!
Hi Jed, just wondering if you could clarify – in this talk you talk about volatile anaesthetics in infants, and mention that they have a lower FRC. My understanding is that soon after birth their FRC comes to near adult values at 30 mL/kg.
Do you mean a lower absolute value for FRC?
Either way the higher minute ventilation, with larger RR would explain the increased speed of induction.
Thanks for the comment. I’ve found some conflicting reports about exactly when the FRC on a ml/kg basis comes up to adult values. I think the most important thing is that their minute ventilation to FRC ratio is much greater than an adult’s and that leads to faster induction.
Thank you so much for your pod cast. Im a dental anesthesia resident, and have found it impossible to learn the material with out guidance. My program doesnt offer any lectures nor structure to the didactic portion of the porgram. Do you by any chance have any porwerpoints, notes, book suggestions, to use in addition to you pod casts. Do you have anything more available than listed on the site? Any help or response would be greatly appreciated. Thank you very much!
I’m glad you are enjoying the podcast. In terms of other resources there are the basic textbooks like Baby Miller (the basic version of Miller’s Anesthesia). University of Kentucky has some great YouTube lectures available by searching YouTube for University of Kentucky anesthesia. I hope that’s helpful. Good luck with your ongoing training!
Dear Jed, I am an anaesthetist in India with about 15 years of experience. While searching for a BJA podcast I stumbled upon yours. I must say this was a fortuitous discovery. I listen to your podcasts while commuting to work in the morning and they set the mood for the day. Have you come across any literature for anaesthetic considerations for surgeries of long duration?
Keep up the good work! And thank you.
I’m so glad you found the podcast and are finding it useful. You ask a great question. I’m not aware of any studies on that specifically. I think in general you would want to be very careful with any agents with long context sensitive half-lives, at least if you were planning on waking the patient up and extubating at the end of the case. For long cases, especially with obese patients, we may use sevoflurane or desflurane instead of isoflurane, or if we use isoflurane we would turn it down early. We might use remifentanil instead of fentanyl as well. If we use a propofol drip we would be very conscious of turning down the dose as the hours go on. Let us know if you have any other thoughts or if you come across any literature on the subject. Thanks!
I just wanted to thank you for doing these podcasts. I did catch one little error in this one though. I know this one more than a couple years old at this point, but when you are speaking of N2O you stated that it is highly soluble. As far as I understand, N2O is not very soluble but highly diffusible. Just wanted to make sure any new-comers such as myself didn’t get the wrong idea. Thanks again!
You are correct. Nitrous is an insoluble gas in blood just like desflurane. Thanks!
I was wondering if you could help explain how the Fa/Fi ratio is impacted by cardiac output and shunting?
I know in the podcast you mentioned that increasing cardiac output will delay induction. I would imagine that increasing cardiac output will have less of an impact on insoluble agents since less of the inhaled agent is taken up by the blood? Is the same logic true for patients who are anemic? If patients are anemic, they will have a delayed onset and insoluble agents will be impacted less so than soluble agents?
I am a little confused when talking about the right-to-left shunts. I know you mentioned that this r-l shunt will delay induction and this will be more prominent for insoluble agents since there is already very little agent in the blood. Thereby the shunt will dilute it even more. What confuses me is that you mentioned to think about the alveolus / how the alveolus reflects the brain concentration…. So how does thinking about the amount of anesthetic dissolved in the blood relate to the amount that is in the brain? Because if I think that this shunt will delay onset since less agent is in the blood, then I would want to think that increasing cardiac output would increase induction since more anesthetic is dissolved in the blood.
Thanks for the help!
You are correct that increasing cardiac ouput has the greatest effect on more soluble inhaled agents. Anemia, in and of itself, isn’t something I think of as having an effect. It may, but I’m not sure why.
Right to left shunts dilute out the anesthetic in the blood. Increasing cardiac output does not increase the amount of anesthetic dissolved in the blood, it decreases the amount in the alveoli and the brain by carrying it away faster and allowing it to equilibrate with other compartments (muscle, fat, etc).
I hope that helps!
Thanks for the comment!
So with a right to left cardiac shunt, the anesthetic is diluted in the blood… But why do we care about how much anesthetics is in the blood?
I thought the important thing was how much anesthetic is in the alveolus, not how much is in the blood since overall we are concerned about Fa coming to equilibration with Fi for induction to occur? So where does the amount dissolved in blood come into this? Thanks so much for the help!!
If you dilute the concentration in the blood it will create a larger gradient for the anesthetic to leave the alveolus and go into the blood, so it will take longer for the alveolus to reach steady state.
i decided to give european Board exam. and i made a research and i stumbled to your podcast. i appreciate your works.
You opened a different way in brain to use information 🙂
Thank you so much
Thanks so much for the note! Good luck on your exam!
I understand the second gas effect concept, just confused when you said Nitrous Oxide is taken up so quickly (more than sevo) despite its lower blood:gas coefficient (hence solubility)?
Isn’t it the less soluble an agent, the less uptake it has?
Great question. This is a very confusing concept. Nitrous oxide is relatively insoluble. But it is much more soluble than nitrogen and given the concentration gradient even more than oxygen. So it will leave the alveolus faster and the concentration of other gasses will be higher than they would have been if you were using air or oxygen instead. I hope that helps!